I have an IBM Thinkpad T20, circa 2000, that a few months ago stopped booting, and instead beeps four times in quick succession and then turns off. I looked at the IBM documentation which suggests a faulty motherboard.
However, I'm wondering if it is something as simple as the CMOS battery needing replacement. Would a flat CMOS battery cause the four beep code to occur?
When my ThinkPad 600e had a CMOS battery issue, it still booted. Now, it is a couple years older than your T20, and I'm willing to be wrong, but I don't think the CMOS battery will prevent a boot. It'll just cause an extra error during POST.
I would try reseating the RAM, and maybe the CPU if you can get at it
If you have two sticks of RAM, try each one on it's own
Here's the Thinkpad T20 service manual: support page, direct pdf link. The manual also covers the T21/22/23 models.
OP, grab a copy of this (although it sounds like you already have it).
You might get lucky and fix your issue by replacing the CMOS battery. If not, this service manual will help you do a partial/complete teardown of the system. You'll want to first try reseating the connectors for all the peripherals (RAM, network, drives); if that doesn't help, try to get to the CPU and LCD connectors.
From there, try disconnecting the non-critical components -- network and drives particularly. If you're still not getting anywhere, try swapping in parts if available, particularly RAM and CPU.
If you get to this point, you need to examine whether it's worth trying to fix. You might be better off finding another T20 (dead or alive) and try combining parts from both machines into one working system.
Take the battery out, with the power off. Put it back in and power up. Fixed my problem. If this doesn't work, turn the laptop off, take the battery out and power up without it. My T20 had this exact problem. 4 beeps and no display. Try it! This nothing listed for 4 beeps in any Thinkpad beep code listing I've seen.
